It's more complicated than that. You can see what passes are run by clang with the flags -mllvm -debug-pass=Arguments. It's certainly true that -O0 involves few

It's more complicated than that. You can see what passes are run by clang with the flags -mllvm -debug-pass=Arguments. It's certainly true that -O0 involves fewer passes than -O1 which involves fewer passes than -O2. But the speed limit is defined by the architecture. A compiler that was engineered for compilation speed with a goal of -O1 tier code quality would have a very different architecture from day 1 (e.g. unified IR, fewer passes, integrated register allocation and code generation). There's been attempts at "punching through the layers" over the years with things like fastisel, but there are limits. That said, all the major compilers can deliver adequate compilation speed on the order of 50-100 kloc/sec for well-structured C code bases. Speed is much more of a serious concern if you're working on large C++ code bases. Or if you're using LLVM as a backend from a language like Julia which does the moral equivalent of C++ template instantiation at runtime and feeds it to LLVM.